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Old 22nd Sep 2020, 2:29 pm   #21
AdrianH
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Default Re: Question on EF91/EF80 and old TV's

Points noted Andy;

I was a service engineer but generally PMR radio gear, before early retirement my dealings with valve equipment would have been in the previous century.

I have gone for cheap and a desktop model £20 for the Ferguson 991T, I will collect when I can and save in the garage away from eyes for a little while. In the mean time I will await the modulator and see if I can gain a mains isolation transformer.

Cheers

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Old 22nd Sep 2020, 3:04 pm   #22
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Default Re: Question on EF91/EF80 and old TV's

There is an unnecessary fear of 'live chassis' sets....they are only live if you connect them that way....make sure chassis is connected to neutral. Incidentally an isolation transformer doesn't make the set any safer to work on, HT is still HT, EHT is still EHT and you'll get a belt if you touch the wrong bits whether it's isolated or not.

What an isolation transformer allows you to do is connect earthed test equipment.

Think of the really old days back in the 60's and the TV engineer that used to repair sets in customer's homes. No isolation transformers, possibly only a two-pin socket (or a light socket in some cases)! Back off in a dimly lit room and just an Avo, a box of valves and some common spares. The wary might carry a neon screwdriver to check if the chassis was live and reverse the mains plug if necessary then it's fault-finding as usual. In my 35 odd years in the trade and doing just the above (in the early days) I don't recall one fatality from a non-isolated (I prefer that term) set.

What about the 70's and 80's when 'half-mains' chassis became popular? It didn't matter what way round the mains was connected, you had about 120V on the chassis. these were serviced in customer's homes again without an isolation transformer (although I think some of the rental companies supplied their engineers with a portable transformer). Otherwise it was repaired in the same way as the early 60's sets. Again I don't recall any fatalities from working on these sets 'live'.

I'm only recalling all this because 'live' chassis was the norm for UK sets. Of course you have to be careful but that applies to anything mains. By all means use an isolation transformer (you'll need a 500W type) but it really doesn't make the set any safer to work on unless perhaps you are standing with bare feet on a concrete floor.....!
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Old 22nd Sep 2020, 4:27 pm   #23
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Default Re: Question on EF91/EF80 and old TV's

The Ferguson 991T is very interesting but very complicated as a starter. Early super fringe model with flywheel sync and high gain I.F. stages. I think the chassis sports 22 valves!

TV chassis that were equipped with a mains ISOLATION transformers are very rare and usually had the EHT supplied by a mains transformer and you don't want to get involved with that! Large numbers of KB, Plessey, Ekco, Ultra including many more had a mains connected chassis that supplied H.T. and a transformer for valve and tube heaters only.

There were some odd arrangements and care has to be taken. [Watch out for early Murphy's]

It's not a problem working on a live chassis providing steps are taken to insure that the chassis is always connected to neutral but the use of a isolating transformer is a much safer procedure. John.
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Old 22nd Sep 2020, 4:32 pm   #24
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Default Re: Question on EF91/EF80 and old TV's

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I have gone for cheap and a desktop model £20 for the Ferguson 991T, I will collect when I can and save in the garage away from eyes for a little while.
That's a good set to start with. I wouldn't keep it in the garage for too long, especially with the colder weather coming, any damp could cause problems with the line output transformer. I don't know about you, but I find the trouble is coming up with a convincing excuse to put another TV or radio in the house in order to get permission from SWMBO.

Cheers
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Old 22nd Sep 2020, 5:17 pm   #25
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Default Re: Question on EF91/EF80 and old TV's

The idea of the isolation transformer for me is to stop any RCCB trips in the house and to allow me the chance to use items like my scope, I have no particular wish to remove earth's from gear before hand. Yes I appreciate in some circumstances it can make things a bit more prone.

I have to shift a few things around and dump my AR77 project as I really have no will to continue with it. I have other receivers for SW.

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Old 23rd Sep 2020, 11:51 am   #26
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Default Re: Question on EF91/EF80 and old TV's

Interesting you mention this set- there’s one on eBay at the moment in Norfolk, I’d be tempted if I had the space. I was puzzled by the power supply chassis- LOTS of valves, but only one large mains transformer and a smoothing choke, which puzzled me. A bit of googling suggested that it actually used a flyback derived EHT using two valve rectifiers in a doubler arrangement. Looked like they were fed from their own o/p valve on the PSU chassis too, although I might have got that wrong.

A bit like the old trick of hanging a tripler off the o/p valve, although done with more valves. Presumably much more expensive than a simple overwind.

So fairly suitable for the OP, except perhaps on the count of value and scarcity..

Oliver[/QUOTE]

Thanks for the 'heads up' on that one Oliver I hadn't seen it. That's not actually the one I was thinking of, my memory has failed me again, although its from about the same period - not seen one of those since the '50s.

The one we serviced was a 9" table model. and I think was all on one chassis although the layout looked similar. It was the first set I ever saw with 'picture shift' controls, it fed a DC voltage into one end of the line and frame scan coils and you could adjust the position with two pots.

It had a something like 60v winding on the mains transformer to provide the shift voltage, on our set this winding had gone o/c, the cost of a new transformer was prohibitive so our senior engineer fitted a 66 volt press stud battery (if anyone remembers those) into the circuit to bring the picture back on the screen again.

I think it had to be changed about every 6 months but saved a set that would otherwise been scrapped. I do seem to rember that it had two EHT rectifiers so maybe it was the same circuit.

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Old 23rd Sep 2020, 12:34 pm   #27
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Default Re: Question on EF91/EF80 and old TV's

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Well Vidor did make a table set with a transformer, but it is very rare and has mains EHT.
Yes it certainly has! I posted an article on the Vidor 369A a good while back. It is a very good performer but the mains EHT is dangerously exposed and is definitely a none starter.

I plugged it in a few moments ago and it worked without adjustment!
Hope you have success with the 991T. The flywheel sync is very stable once you have evicted the leaky .01uf caps in the discriminator circuit. It has a large component count and you will need quite a few caps. Regards, John.
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Old 23rd Sep 2020, 2:10 pm   #28
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Default Re: Question on EF91/EF80 and old TV's

Time will tell, the 991T is in the back of the car waiting for the rain to stop. I will give a brush off when I can and take things from there.
I have the service sheets courtesy of the BVWS DVD, and a few articles from the Practical television magazines from around 1954 up.

And I will shout help if needed.

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Old 23rd Sep 2020, 7:25 pm   #29
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Default Re: Question on EF91/EF80 and old TV's

Hi Adrian, if it all turns pear shaped and you still have a small tube with scan coils, try one of the early, non VCR97 PTV designs.
You could even cheat and use a set top box for audio and video and avoid the hassle with RF stages

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Old 23rd Sep 2020, 7:45 pm   #30
AdrianH
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Default Re: Question on EF91/EF80 and old TV's

I was seen with the TV by the wife and received, the look!

On the TV front, I have dusted it down and cleaned the crud from the tops of the valves a bit all in prep for bring it inside at some point, I have a few issues with it but at £20 what can I expect. What I shall do is close this thread and start another thread as my foray into old TV's.

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